Field of the Invention
The present invention relates to a copying zoom lens system that is capable of zooming with the object-to-image distance being held constant and which hence is suitable for use in the optics of a copying machine.
2. Background of the Invention
Descriptions of a copying zoom lens system of the three-lens-unit type contemplated by the present invention are disclosed in Unexamined Published Japanese Patent Application Nos. 67909/1982 and 121414/1985. The systems taught in these applications consist of 8 to 10 lens elements and attain half viewing angles of the order of 18.degree.. Since these system are expensive and require a long object-to-image distance, they are not capable of fully meeting the requirements of modern copying machines for lower cost and smaller size.
The assignee of the present application previously filed Japanese Patent Application No. 26419/1984 in which they proposed a copying zoom lens system that was intended to satisfy the requirements for smaller size and lower cost. This system is of the seven-element configuration and encompasses a half viewing angle of the order of 25.degree.. However, even this system is not completely satisfactory since the half viewing angle that can be attained is not on the order of 30.degree.. Furthermore, its configuration wherein the second and third lens elements are made separate from the fifth and sixth elements is prone to misalignment between the axes of lens elements being assembled.